1. Field of the Invention
The present invention relates to a battery pack and a battery pack stack.
2. Description of the Related Art
In general, a battery pack used in portable external electronic devices, such as notebook computers, personal digital assistants (PDAs), and camcorders, is fabricated by combining a number of battery cells into a single unit due to the limited capacity of just one battery cell. Furthermore, a plurality of battery packs, electrically connected to each other in a stacked structure, are used in devices, such as electric vehicles and uninterruptable power supplies (UPSs), which have larger capacity requirements than portable electronic devices.
When a battery pack accommodates a plurality of battery cells which are electrically connected to each other in parallel, both a positive electrode and a negative electrode are extended from a battery cell at one side and electrically connected to an external device. In the same manner, when a battery pack stack includes a plurality of battery packs which are electrically connected to each other in parallel, both a positive electrode and a negative electrode are extended from a battery pack at one side and electrically connected to an external device.
However, when a battery pack or battery pack stack connected to an external device is charged or discharged, the charge/discharge rate of the battery cell or battery pack disposed on the side where the positive and negative electrodes are extended is higher than that of the battery cell or battery pack disposed on the opposite side. Thus, the amount of heat generated from the battery cell or battery pack at the side where both the positive and negative electrodes are extended is greater than the amount of heat generated from the battery cell or battery pack at the opposite side. In this state, as the number of charge/discharge cycles increases, degradation of the battery cell or battery pack at the side where both the positive and negative electrodes are extended becomes serious. Thus, discrepancies in service life arise between the battery cells or battery packs.